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My journey into the world of solar cooking.

Solar Box Oven: an easy DIY project

The amount of documentation available on the ‘net these days about solar ovens & cookers is absolutely staggering. People have been messing around with solar cooker design literally for centuries – yes, I mean hundreds of years. So go ahead and search the ‘net and browse away. Just set aside a specific time for research before going hands-on. If you don’t, you could spend months reading all the pros and cons of various designs and not get around to actually making something. Here are two sites to get you started:solarcooking.wikiabuilditsolar.com

After the first hour of reading, stand up and walk around your basement, attic, storage shed, anywhere you stash your stuff and look around for items that might be useful in a solar cooker. By now you’ll know what many of these things are, but don’t worry about it. Even stuff you don’t think will be useful may turn out to be needed for some crazy design you decide you want to try. Keep an open mind.

Since I had decided on a box oven design that would mimic the commercially manufactured Global Sun Oven (GSO, see photo on left side of my header), the first thing that I looked for was a sturdy box that fit the dimensions spec’d on some website:

“The GSO inside is cut and either machine stamped or machine formed out of sheet aluminum. It is anodized black inside. 17″ X 17″ square, with the top sloping with the front (low end) 9″ high and the rear (high end) 12″ high.”

I got lucky and quickly found an old laser printer box that measured 18″ x 18″ square with 13″ high sides. I also had plenty of large sheets of plain cardboard that could provide multiple layers for insulation. I’d seen people stuffing things like newsprint or perlite between cardboard “walls”, but I decided to keep it simple and add two layers of cardboard on the inside of the thick-walled box.

But I’m skipping ahead a bit. My first decision was to make the angle of the glazing (the window) steeper than the GSO’s. Instead of a 9″ front wall, I made mine ~6″. I left the back wall the full 13″ height of the box. This resulted in a glazing angle of ~23º when the box was flat on its bottom.
In cutting down 3 sides of the box to get that shape, I set the box flaps aside to use as support for the floppy (note bamboo pieces used as support) aluminum-lined Reflectix that I decided to use for reflectors. It’s what I had lying around. Coroplast (plastic cardboard) lined with reflective mylar would definitely be better, but I didn’t have those and I didn’t want to wait to get them.

The Reflectix I had was 24″ wide, so I cut four 18″ sections and glued 3 of them to the 3 box flaps. I velcroed the 4th section to the box flap that was still attached to the box. One by one I clipped the ends of the Reflectix together until I got a rough approximation of the angles of the GSO reflectors. I can adjust it as needed.
I lined the inside walls with plain old aluminum foil – stuff I had in the kitchen. I chose not to use glue since it might stink up the box, so I just folded it around the edges and wedged it in as best I could. It’s not perfectly flat, so watered down Elmer’s glue might’ve been a better option. I also poked holes through the foil and all the cardboard layers, then threaded an unfolded paperclip through everything to “sew” the loose cardboard sheets to the main box. I used 2 paperclips per side. I have some binding rivets somewhere that would do a neater job, but I couldn’t find them.

For the glazing I had a couple of choices. I had some fairly thick (3/16-1/4″?) glass that a glass shop a few blocks away could have cut to size for $5-10, but I decided not to risk the common glass cracking issues that many people had reported. Instead I went with 1/8″ polycarbonate, a Lexan substitute. Fortunately there’s a Tap Plastics nearby and it only cost ~$18 with tax.
The day after I finished making the box oven I put an empty black graniteware pot in the oven and set up the reflectors at 9 am. It was a clear day with highs in the low 80s. Within
30 min. the oven temp was 275ºF and stayed there until I opened the oven to add the ingredients for beef stew into the pre-heated pot at 10:40 am.
I rotated the oven to follow the sun about once an hour. The oven temps stayed in the 225-265º range. The wind occasionally picked up and knocked my rickety reflectors askew, so I had to adjust those a few times.

I tasted the stew after 5 hrs. The meat, mushrooms and carrots were nicely done, but the potatoes and onions needed just a bit more time. I had decided to use medium sized potatoes & onions whole instead of quartered or sliced. Another hour in the oven helped, but I think I’ll slice those ingredients next time.

I have a few refinements in mind to try and get higher oven temps, but for the next few months this will function very well as a slow cooker.